1. Field of the Invention
The invention relates to a system for the early detection of a change in drilling mud flow rates in an operational drilling rig and, more particularly, the rapid detection of an impendent "blow-out" condition or an impendent "lost circulation" condition during operation of borehole drilling equipment.
2. History of the Prior Art
In the drilling of an oil and gas well, a common, ever present danger is that of drilling into an earth formation which contains high pressure fluids. When this occurs, the high pressure fluids from the formation enter the borehole and displace the drilling mud back up the borehole toward the drilling rig at the surface. If the intrusion of high pressure fluids back into the borehole is not detected quickly and controlled, it can result in the complete displacement of the drilling mud back up the borehole and expulsion of the high pressure fluids out the top of the borehole. This event is called a "blow out" and can result in great injury to both property and life due to the high combustability of the natural gas and other fluids and the violence with which they exit from the borehole.
On the other hand, it is possible that, during drilling, a borehole may enter a formation which is highly porous and create a tendency for all of the drilling mud to flow freely from the borehole into the porous formation. This event is termed "lost circulation" and can result in the substantial loss of drilling fluids if the lost circulation is not detected very rapidly and preventive measures taken. Upon the impendence of either of these two events, "blow-out" or "loss circulation", it is desirable to detect them as rapidly as possible in order to take remedial action to control the run-away mud flow and prevent either its substantial loss into a porous formation or to prevent its moving back up the borehole toward the surface and thereby prevent the possibility of personal injury and damage to equipment resulting from that rapid upward movement.
It is known to compare the input mud flow with the return mud flow in a borehole. A substantial increase in the rate of return mud flow with no corresponding increase in input flow is indicative of a "blow-out" whereas a substantial increase in input flow without a corresponding increase in the output flow of mud is indicative of lost circulation. The biggest prior art difficulty with measuring these changes in mud flow rates has been that it has only been possible to make such measurements near the surface end of the borehole. Therefore in a deep borehole it was only possible to detect the imminence of "blow-out" at a location which is quite remote from the location down in the borehole where the event actually occurred. Thus, substantial amounts of time may have elapsed prior to the detection of the occurrence of this event at the surface and substantial damage may have also occurred before remedial action can be taken.
One additional difficulty associated with the surface measurement and detection of sudden changes in mud flow rates is that offshore wells are often drilled by either a drilling ship or a floating platform. Therefore, compensation must be made in the drill string for the movement of the drilling support structure floating on the surface of the sea. With the waves and tides associated with such offshore drilling operations, it is necessary to provide telescoping sections in both the drill string itself as well as in the riser pipe, which conducts the return mud flow from the top of the borehole in the sea floor back to the vessel. As the riser pipe telescopes in and out with wave action, the volume of mud contained within the riser pipe section varies in accordance with the telescoping movements. Thus, if mud flow measurements are made on the drilling vessel at the surface, this periodic and cyclic variation in the mud flow must be compensated for when attempting to measure a rate of change of mud flow which might be occurring due to "blow out" or "lost circulation" conditions occurring down hole. Most prior art techniques have measured mud flow rates at the surface and have, therefore, concerned themselves principally with compensating for variations in mud flow due to the telescoping action of the riser pipe.
Techniques for monitoring an impending blow out by down hole measurement have generally relied upon the detection of gas influx into the return mud flow path in the annular region between the drill pipe and the walls of the hole. The measurements of such a parameter in the return mud flow is only indicative of gas kick and is virtually ineffective to alert the drilling crew to a lost circulation condition.
The system of the present invention overcomes many of the disadvantages of the prior art by making the measurement of mud flow rates down hole near the drilling bit where an actual change of relative mud flow rates first occurs under either a gas kick or lost circulation condition. Also, the system of the present invention avoids the necessity of making substantial and complicated calculations to compensate for riser pipe volume variations as would be necessary when the measurement is made near the surface where the mud flow enters and leaves the riser pipe.
The system of the present invention functions very effeciently to rapidly measure the rate of change of input mud flow and output mud flow near the bottom of the borehole and detect any substantial variation in those flow rates to interpret those variations and produce an alarm signal upon the occurrence of either a blow-out or lost circulation condition.